Sarah joined the Brotherhood of Steel because her brother, Edward, became a member. When their parents died, Sarah promised to look after her younger brother and take care of him. If this meant following him into the Brotherhood, she was more than willing to do so.
Sarah disliked the military and just couldn't seem to excel at any of the skills required to get ahead. Everyone knew that she was there because of Edward. And everyone also knew that there was no getting rid of her unless her brother went with her. So, everyone found something for Sarah to do until she took an interest in the medical field. Much to everyone's surprise, including her own, she showed the potential to become a competent doctor. Sarah's place in the world had been discovered.
Sarah is currently an intern at the base. Unfortunately for her, the former staff deserted and she has been left to further her career on her own. She takes her work seriously, and is studying everything she can find related to medicine in the bunker's library.
Cure the commander: This quest starts indirectly by talking with Sarah. If the Prisoner has a high enough Medicine skill then he can choose to aid Sarah solving this quest initially and gain immediate access to level 2 of the bunker.
After the Prisoner is found investigating the entrance to the bunker he will be briefly questioned and invited in, where he will be taken to the medical center for disease testing. This is a scripted scene where the Prisoner will become locked in dialogue with Sarah. The guards will leave the room and talk will commence. Cooperating with Sarah will lead to her revealing critical plot information and giving the Prisoner limited access to the base. Non-cooperation causes her to call in the guards who will escort the Prisoner out of the base and refuse to let him enter again.